With recent development of functional materials, there are provided, for example, materials such as microcapsules of which shape can be maintained only in a liquid medium. For example, it has been proposed an ink for ink jet recording containing a microencapsulated functional substance to improve color developing properties and weather resistance. In such a case, it is very important to do analysis and evaluation of the microcapsules on shape, composition etc. thereof to know the ink properties.
One of the conventional evaluation methods is scanning electron microscopy. Japanese Patent Application Laid-open No. H09-057091 discloses a method for observing a cross section of microcapsules including a volatile substance by using an ordinary scanning electron microscope.
In case of microcapsules dispersed in water, the above method first separates the microcapsules from water and observes the inside of the microcapsules having a resinous shell. Thus, the real shape of the microcapsules in a liquid medium in actual use may not be obtained by such a method.
In general, a sample of high molecular substance holding its shape in a liquid medium such as water, for example, a water-containing sol or gel, loses its structure when placed in a scanning electron microscope due to water evaporation. In addition, such a sample has a high electrical resistance without electrical conductivity, so that it is charged up and a secondary electron image cannot be obtained.
Therefore, such a sample is observed and analyzed most suitably by using an environment-controlled scanning electron microscope.
Such environment-controlled scanning electron microscope is also called a low vacuum scanning electron microscope, where the observation is carried out under a low (poor) vacuum level in comparison with the ordinary scanning electron microscope.
In such a low vacuum scanning electron microscope, however, a residual gas remaining in the chamber due to the low vacuum level hinders flight of the secondary electrons generated from the sample to a detector, whereby a detected signal level becomes low and a sufficient resolution cannot be obtained.